Detective Comics #312

Matt Hagen breaks jail again and uses his pool of protoplasm to renew his Clayface identity. Using his powers, Hagen is unstoppable, until Batman follows him to the secret grotto where the pool is located. Batman and Hagen start fighting and both of them fall into the pool, after which they are both transformed into clay. Batman uses his new powers to become a Clayface-Batman in order to fight Clayface on even terms. At the end of the fight, Batman defeats Clayface and returns him to prison.